This invention relates to arrays of detectors of incident radiation for obtaining imaging data of a radiant subject and, more particularly, to a focal plane array of photodetectors employing rows of readout diodes and charge transfer gates constructed upon a common substrate with the readout diodes and the gates, wherein the diodes are coupled via the charge transfer gates to the photodetectors, this providing improved resistance to damage from high energy radiation including blasts of gamma radiation.
Imaging systems for obtaining image data of subjects by viewing radiation in the ultraviolet and/or visible range of the electromagnetic spectrum may employ an array of radiation detectors permitting electronic readout of imaging data. By way of example, such systems may be employed in outer space for viewing stars for celestial navigation, as well as for tracking moving vehicles. Such an imaging system may include a lens for focusing incident radiation upon a focal plane array of detectors for detecting pixels of the image. Each detector produces an electric signal, such as an electric charge, in response to illumination by the radiation. The detector array is provided with electrical paths for conducting signals from the detectors to an image processor which converts the detector signals into an image suitable for viewing by a person or for operation of automated electronic navigation and/or tracking equipment.
One form of detector array in present use employs charge-coupled devices (CCD's) arranged in rows and columns. Electric circuitry for use with CCD's is well known, and provides for a construction of the array wherein charges produced at the sites of the pixels in each row of CCD's are clocked to the end of the row to provide a sequence of pixel signal providing data of a row of the image. The pixel signals from the various CCD rows of the array are multiplexed for communication to image-processing equipment to obtain the imaging data. Addressing of the various pixels is obtained by timing circuitry which provides for a sampling of each pixel signal at a designated sample time which serves to identify the location of each pixel in the detector array.
A problem arises in a situation wherein the detector array may be subjected to intense fields of nuclear radiation, particularly high-energy x-rays or gamma radiation. Degradation in the performance of the CCD detector array from exposure to the radiation is manifested in a degradation in charge-transfer efficiency, the generation of increased dark current, and the possible development of shorted and/or disconnected elements of the detector array. Electronic devices which have substantial immunity to high intensity radiation are said to possess radiation hardness. The foregoing areas of sensitivity of CCD detector arrays to radiation fields limits the application of conventional CCD detector arrays in the case of hardened systems.
A particularly disadvantageous feature of a CCD array, from the point of view of radiation hardness, is the serial transfer of charge along a row of pixels from one pixel to the next. A breakdown in one of the charge transfer elements from intense gamma radiation may disable a major portion or possibly an entire row of the detector array. Another form of construction of the foregoing array employs CID's (charge injection devices) wherein each column electrode is connected to all pixel detectors along a column of detectors. This form of construction avoids the disadvantage of the CCD wherein the charges in a row of detectors must propagate through numerous charge transfers before being extracted from the array. However, in the CID array, the connection of numerous detectors of the column to a single output column electrode introduces a significant increase in the output capacitance of the array. The increased capacitance limits the array size in the column direction so as to avoid excessive capacitance. It is noted that, in the rapid readout of successive signals from the detectors of an array, the amplitudes of the signals decrease with increasing capacitance, hence there is need for reduction of the column length in a CID array. Furthermore, CID arrays are susceptible to negative blooming and loss of contrast due to charge integration in all pixels of a column upon a reading of the signal of a single pixel. Integrated charge in other pixels is generated by incident high-energy photons, ionizing radiation, or by dark current resulting from the radiation-produced degradation of minority-carrier lifetime. Thus, neither the CCD or the CID form of construction of detector array is adequately suited for a radiation-hardened imaging system, employing a large detector array.